This topic is a how to.Please keep it as clear and simple as possible. Avoid speculative discussions as well as a deep dive into underlying mechanisms or related technologies.

As a SharePoint Power-User or a SharePoint Administrator depending on responsibility policy made, you sometimes are asked regarding uploading multi- document. Or getting involved to SharePoint community, you would have seen a lot of questions as to grayed
Upload Multiple Document functionality.

When opening SharePoint document library, you would be asked to run Microsoft Office 2010 component by Internet Explorer automatically.

After allowing, I’ve checked and noticed the ActiveX Control called STSUpld.UploadCtl (http://msdn.microsoft.com/en-us/library/ms456628.aspx) in the
Manage Add-ons option is enabled.

After that, check Upload Multiple Document functionality, you would see that it works well.

If you disable STSUpld.UploadcCtl, Upload Multiple Document functionality will be grayed out.

According Microsoft, this control is defined in the STSUPLD.dll, a dynamic-link library (DLL) that is installed on the client computer in the
%ProgramFiles%\Microsoft Office\Office14\ directory during Microsoft Office 2010 Setup.

In my humble opinion, this basically means if your computer doesn’t have STSUPLD.DLL stored in
%ProgramFiles%\Microsoft Office\Office14\ directory, you can not use Upload Multiple Documents functionality.

If you are using Microsoft Office 2007 on WSS 3.0 or MOSS 2007 environment, STSUPLD.DLL is stored in
%ProgramFiles%\Microsoft Office\OFFICE12\ directory.

In a nutshell, if your end-user can not upload multiple documents into a document library, make sure the following things at least: